2,4,5-trichlorophenoxy alkanoic acids, especially, 2,4,5-trichlorophenoxy acetic acid (2,4,5-T) and 2-(2,4,5-trichlorophenoxy) propionic acid (Silvex) and the salts, aliphatic esters and amides thereof are known to be particularly effective herbicides.
2,4,5-trichloropenoxyalkanoic acid, e.g., 2,4,5-T is typically prepared by condensing 2,4,5-trichlorophenol with haloalkanoic acid e.g., monochloroacetic acid in aqueous alkali metal hydroxide solution as described for example in U.S. Pat. No. 2,598,692. It is also known to prepare 2,4,5-trichlorophenoxyalkanoic acid, e.g., 2,4,5-T by chlorination of the sodium salt of 2,5-dichlorophenoxy acetic acid as described, for example in U.S. Pat. No. 2,717,907.
The use of both 2,4,5-T and Silvex has been severely curtailed by a recent order of the Environmental Protection Agency since each of these materials contain trace amounts of chlorinated dibenzo-p-dioxins, particularly 2,3,7,8-tetrachloradibenzo-p-dioxin (2,3,7,8-TCDD) which is produced as a by-product in the production of 2,4,5-T and Silvex.
2,3,7,8-TCDD is extremely toxic and there is evidence that it can be fatal to certain species of laboratory animals at levels as low as 0.6 microgram per kilogram of body weight.
Methods have been devised for treating 2,4,5 or Silvex to reduce their 2,3,7,8-TCDD content which methods are described, for example, in U.S. Pat. Nos. 4,026,917 and 3,840,593. In U.S. Pat. No. 4,026,917 dioxin is removed from 2,4,5-T by adsorption of the dioxin with coconut charcoal. This method is reported to reduce dioxin content of 2,4,5-T to less than 1 part per million. In U.S. Pat. No. 3,840,593 Silvex is subjected to fractional liquid-liquid extraction with a polar liquid solvent which reportedly reduces the dioxin content of the Silvex to typically less than 0.1 part per million. However, such treatment methods though removing a portion of the dioxin from 2,4,5-T or Silvex pose the problem of disposing of the dioxin-contaminated treating material, i.e. the charcoal in the case of U.S. Pat. No. 4,026,917 and the solvent in the case of U.S. Pat. No. 3,840,593. In other words such methods address only the symptoms rather than the cause. Moreover detectable amounts of dioxin remain in the treated 2,4,5-T and Silvex.
Prior to this invention, trichlorophenoxy alkanoic acids, e.g. 2,4,5-T, Silvex and hydrolyzable derivatives thereof free of detectable amounts of chlorinated dibenzo-p-dioxins, particularly 2,3,7,8-TCDD, have been unknown to the art, since previous processes have reputedly produced product containing varying quantities albeit in some cases small quantities of chlorinated dibenzo-p-dioxins.
For example, current manufacturing processes are believed capable of producing 2,4,5-T containing about 0.01 parts by weight of 2,3,7,8-TCDD per million parts by weight, 2,4,5-T. Although 10 parts per billion 2,3,7,8-TCDD in 2,3,4-T might ostensibly appear, for all practical purposes, negligible, due to the extreme toxic nature of 2,3,7,8-TCDD, material containing even such low levels is considered potentially hazardous.